In this episode of Virus Watch, I explain how mosquitoes spread viruses. We’ll look at how a mosquito finds a host, how it finds a blood vessel, and how it delivers viruses to a new host. Don’t blame mosquitoes for viral diseases: it’s not their fault!

Not long after the appearance of an outbreak of viral disease, first scientists, and then newswriters, blame it all on mutation of the virus. It happened during the Ebolavirus outbreak in West Africa, and now it’s happening with Zika virus.

The latest example is by parasitologist Peter Hotez, who writes in the New York Times:

There are many theories for Zika’s rapid rise, but the most plausible is that the virus mutated from an African to a pandemic strain a decade or more ago and then spread east across the Pacific from Micronesia and French Polynesia, until it struck Brazil.

After its discovery in 1947 in Uganda, Zika virus caused few human infections until the 2007 outbreak on Yap Island. The virus responsible for this and subsequent outbreaks in Pacific Islands is distinct from the African genotype, but there is no experimental evidence to suggest that sequence differences in the Asian genotype were responsible for the spread of the virus. For this reason I disagree with Dr. Hotez’ conclusion that mutation of the virus is the ‘most plausible’ explanation for its global spread. It is just as likely that the virus was in the right place at the right time to spark an outbreak in the Pacific.

We will never have experimental evidence that emergence of the Asian genotype allowed pandemic spread of Zika virus, because we cannot test the effect of individual mutations on spread of the virus in humans. Consider this experiment: infect a room of humans (and mosquitoes) with either the African or Asian genotype of Zika virus, then measure virus replication and transmission. If there is a difference between the two viruses, engineer specific mutations into the virus, reinfect another batch of humans, and continue until the responsible mutations are identified. Obviously we cannot do such an experiment! We could instead use animal models, but these have limitations in extrapolating results to humans. For this reason we have never identified any specific mutation that allows an animal virus to replicate more efficiently in humans.

The same experimental limitations do not apply to animals. An example is Chikungunya virus, spread by Aedes ageyptii mosquitoes. Before 2004, outbreaks of infection were largely confined to developing countries in Africa and Asia. The virus subsequently spread globally, due to a single amino acid change in the envelope glycoprotein which allows efficient replication in Aedes albopictus, a mosquito with a greater range than A. ageyptii. It was possible to prove this point by assessing the effects of changing this single amino acid on virus replication in mosquitoes. The same experiment cannot be done in humans.

There is no evidence that the Asian genotype of Zika virus is any more competent to replicate in mosquitoes than the African strain. Results of a study of replication of Asian genotypes of Zika virus revealed that Aedes aegypti and Aedes albopictus are not very good vectors for transmitting ZIKV. The authors smartly suggest that “other factors such as the large naïve population for ZIKV and the high densities of human-biting mosquitoes contribute to the rapid spread of ZIKV during the current outbreak.” In other words, don’t blame the Zika virus genome for the expanded range of the virus.

The Zika virus that has been spreading in Brazil, and which has been associated with microcephaly, shares a common ancestor with the Asian genotype. In a recent study of the genomes of 7 Brazilian isolates, there was no evidence that specific mutations are associated with microcephaly. Those authors conclude (also smartly):

Factors other than viral genetic differences may be important for the proposed pathogenesis of ZIKV; hypothesized factors include co-infection with Chikungunya virus, previous infection with Dengue virus, or differences in human genetic predisposition to disease.

It’s easy to blame mutations in the viral genome for novel patterns of transmission or pathogenesis. Viral mutations arise during every replication cycle, due to errors made by viral enzymes as they copy nucleic acids. RNA viruses are the masters of mutation, because, unlike the polymerases of DNA viruses, RNA polymerases cannot correct any errors that arise. As viruses spread globally through different human populations, it is not surprising that different genotypes are selected. These may reflect adaptation to various selective pressures, including different humans, vectors, climate, or geography. There is no reason to assume that such changes influence virulence, disease patterns, or transmission in humans. Whether they do so can never be tested in humans.

Blaming the viral genome is nothing new. At the onset of the 2014 Ebolavirus outbreak in West Africa there were many claims that the unprecedented size of the outbreak was a consequence of mutations in the viral genome. Genomic analysis of isolates early in the epidemic suggested that the large number of infections was leading to rates of mutation not previously observed. This work lead to dubious claims of “Ebolavirus mutating rapidly as it spreads” and Ebolavirus is mutating (Time Magazine). Richard Preston, in the New Yorker article Ebola Wars quoted scientist Lisa Hensley:

In the lab in Liberia, Lisa Hensley and her colleagues had noticed something eerie in some of the blood samples they were testing. In those samples, Ebola particles were growing to a concentration much greater than had been seen in samples of human blood from previous outbreaks. Some blood samples seemed to be supercharged with Ebola. This, too, would benefit the virus, by enhancing its odds of reaching the next victim. “Is it getting better at replicating as it goes from person to person?” Hensley said.

Go back to any viral outbreak – MERS-coronavirus, SARS-coronavirus, influenza virus, HIV-1 – and you will find the same story line. Mutation of the virus is leading to more virulence, transmission, spread. But in no case has cause and effect been proven.

Let’s stop blaming viral mutation rates for altered patterns of virus spread and pathogenesis. More likely determinants include susceptibility of human populations, immune status, vector availability, and globalization, to name just a few. Not as spectacular as ‘THE VIRUS IS MUTATING!’, but nearer to the truth.

On episode #363 of the science show This Week in Virology, The TWiVers reveal influenza virus replication in the ferret mammary gland and spread to a nursing infant, and selection of transmissible influenza viruses in the soft palate.

During breastfeeding, mothers provide the infant with nutrients, beneficial bacteria, and immune protection. Fluids from the infant may also enter the mammary gland through retrograde flux of the nipple. Studies in a ferret model reveal that influenza virus replicates in the mammary gland, is shed in breast milk and transmitted to the infant. Virus may also travel in the opposite direction, from infant to mother.

The role of the mammary gland in influenza virus transmission was studied using a ferret model comprising lactating mothers and nursing infants. Intranasal inoculation of nursing mother ferrets with the 2009 H1N1 influenza virus lead to viral replication and development of influenza in both mother and infant. When the study design was reversed, and 4 week old nursing ferrets were inoculated intranasally with the same virus, viral replication and disease ensued first in the infants, and then in the mothers. Infectious virus was recovered both in the mammary glands and in the nipples at day 4 post infant inoculation, and in mother’s milk from 3-5 days post infant inoculation. Histopathological examination of sections of mammary glands from infected mothers revealed destruction of the mammary architecture.

These results show that nursing infants may pass influenza virus to mothers. It seems clear that influenza virus replicates in the mammary gland and that infectious virus is present in milk. How does this virus infect the mother? One possibility is that infection is transmitted by respiratory contact with virus-containing milk, or by inhalation of aerosols produced by nursing. How influenza virus in the mammary gland would reach the mother’s lung via the blood to cause respiratory disease is more difficult to envision and seems unlikely.

When influenza virus was inoculated into the mammary gland of lactating mothers via the lactiferous ducts, both mother and breast feeding infant developed serious influenza. Infectious virus was detected first in the nasal wash of infants, then later in the nasal wash of mothers. Breast milk contained infectious virus starting on day 2 after inoculation. Histopathological examination of sections from infected mammary glands revealed destruction of glandular architecture and cessation of milk production. This observation is consistent with the results of gene expression analysis of RNA from virus infected mammary glands, which revealed reduction in transcripts of genes associated with milk production.

To determine if human breast cells can be infected with influenza virus, three different human epithelial breast cell lines were infected with the 2009 H1N1 virus strain. Virus-induced cell killing was observed and infectious virus was produced.

Even if we assume that influenza virus can replicate in the human breast, the implications for influenza transmission and disease severity are not clear. Transmission of HIV-1 from mother to infant by breast milk has been well documented. In contrast to influenza virus, HIV-1 is present in the blood from where it spreads to the breast. Most human influenza virus strains do not enter the blood so it seems unlikely that virus would spread to the breast of a mother infected via the respiratory route. However, viral RNA has been detected in the blood of humans infected with the 2009 H1N1 strain, the virus used in these ferret studies. Therefore we cannot rule out the possibility that some strains of influenza virus spread from lung via the blood to the breast, allowing infection of a nursing infant. Some answers might be provided by determining if influenza virus can be detected in the breast milk of humans with influenza.

What would be the implication of a nursing infant infecting the mother’s breast with influenza virus? As I mentioned above, it seems unlikely that this virus would enter the blood, and even if it could, how would the virus infect the apical side of the respiratory epithelium? What does seem clear is that viral replication in the breast could lead to a decrease in milk production which could be detrimental to the infant. If the mother had multiple births, then influenza virus might be transmitted to siblings nursing on the infected mother.

Are you wondering how an infant drinking influenza virus-laded breast milk acquires a respiratory infection? Recently it has been shown that influenza virus replicates in the soft palate of ferrets. The soft palate has mucosal surfaces that face both the oral cavity and the nasopharynx. Ingested virus could first replicate in the soft palate, then spread to the nasopharynx and the lung. A simpler explanation is that nursing produces virus-containing aerosols which are inhaled by the infant.

The individuals who believe that certain types of gain-of-function experiments should not be done because they are too dangerous (including Lipsitch, Osterholm, Wain-Hobson,) cite the 1977 influenza virus H1N1 strain as an example of a laboratory accident that has led to a global epidemic. A new analysis shows that the reappearance of the 1997 H1N1 virus has little relevance to the gain-of-function debate.

Human influenza viruses of the H3N2 subtype were circulating in May of 1977 when H1N1 viruses were identified in China and then Russia. These viruses spread globally and continue to circulate to this day. The results of serological tests and genetic analysis indicated that these viruses were very similar to viruses of the same subtype which circulated in 1950 (I was in the Palese laboratory in 1977 when these finding emerged). Three hypotheses were suggested to explain the re-emergence of the H1N1 virus: a laboratory accident, deliberate release, or a vaccine trial.

Rozo and Gronvall have re-examined the available evidence for the origin of the 1977 H1N1 virus. While there is ample documentation of the extensive work done during the 1970s in the Soviet Union on biological weapons, there is no evidence that Biopreparat had attempted to weaponize influenza virus. The release of the 1977 H1N1 virus from a biological weapons program is therefore considered unlikely.

It is more likely that the 1977 H1N1 virus was released during testing of influenza virus vaccines. Many such trials were ongoing in the USSR and China during the 1960s-70s. C.M. Chu, a Chinese virologist, told Peter Palese that the H1N1 strain was in fact used in challenge studies of thousands of military recruits, an event which could have initiated the outbreak.

The hypothesis that the 1977 H1N1 virus accidentally escaped from a research laboratory is formally possible, but there are even less data to support this contention. Shortly after this virus emerged, WHO discounted the possibility of a laboratory accident, based on investigations of Soviet and Chinese laboratories. Furthermore, the H1N1 virus was isolated at nearly the same time in three distant areas of China, making release from a single laboratory unlikely.

It is of interest that with the onset of the gain-of-function debate, which began in 2011 with the adaptation of influenza H5N1 virus to aerosol transmission among ferrets, the ‘laboratory accident’ scenario for the emergence of the 1977 strain has been increasingly used as an example of why certain types of experiments are ‘too dangerous’ to be done (See graph, upper left). For example, Wain-Hobson says that ‘1977 H1N1 represented an accidental reintroduction of an old vaccine strain pre-1957, probably from a Russian research lab’. Furmanski writes that ‘The virus may have escaped from a lab attempting to prepare an attenuated H1N1 vaccine’. In the debate on gain-of-function experiments, the laboratory escape hypothesis is prominently featured in public presentations.

Rozo and Gronvall conclude that the use of the 1977 influenza epidemic as a cautionary tale is wrong, because it is more likely that it was the result of a vaccine trial and not a single laboratory accident:

While the events that led to the 1977 influenza epidemic cannot preclude a future consequential accident stemming from the laboratory, it remains likely that to this date, there has been no real-world example of a laboratory accident that has led to a global epidemic.

On episode #348 of the science show This Week in Virology, Vincent and Rich discuss fruit fly viruses, one year without polio in Nigeria, and a permissive Marek’s disease viral vaccine that allows transmission of virulent viruses.

A permissive vaccine prevents disease in the immunized host, but does not block virus infection. Would a permissive vaccine lead to the emergence of more virulent viruses?

This hypothesis is based on the notion that viruses which kill their hosts too quickly are not efficiently transmitted, and are therefore removed by selection. However a vaccine that prevents disease, but not viral replication in the host, would allow virulent viruses to be maintained in the host population. It has been suggested that in this scenario, viruses with increased virulence would be selected if such a property aids transmission between hosts.

On the surface this hypothesis seems reasonable, but in my opinion it is flawed. One problem is that increased transmission might not always be associated with increased virulence. The more serious flaw lies in making anthropomorphic assessments of what we think viruses require, such as concluding that increased viral transmission is a desired trait. Our assumptions fail to recognize the main goal of evolution: survival. Evolution does not move a virus along a trajectory aimed at perfection. Change comes about by eliminating those viruses that are not well adapted for the current conditions, not by building a virus that will fare better tomorrow. All the viruses on Earth today transmit well enough, or they would not be here; yet some kill their hosts clearly much faster than others. The fact is that humans have little understanding of what drives virus evolution in large populations. Our assumptions of what constitute the selective forces are usually tainted by anthropomorphism.

This long preamble is an introduction to a series of findings which are purported to support the idea that permissive vaccines (the authors call them ‘leaky’ and ‘imperfect’ vaccines but I dislike both names because they imply defects) can lead to the selection of more virulent viruses. The subject of the paper is Marek’s disease virus (MDV), a herpesvirus that infects chickens. MDV is shed from feather follicles of infected chickens and is spread to other birds when then inhale contaminated dust. Vaccines have been used to prevent MDV infection since the early 1970s. These vaccines prevent disease, but do not block viral replication, and vaccinated, infected birds can shed wild type virus. The virulence of MDV has been increasing since the 1950s, initially from a paralytic disease, to paralysis and death. The authors wonder if the use of permissive Marek’s vaccines has lead to the selection of more virulent viruses.

To address their hypothesis, the authors inoculate vaccinated or unvaccinated chickens with a series of MDV isolates that range from low to high virulence. Unvaccinated chickens inoculated with the most virulent MDV died within a week and shed little virus. In contrast, most vaccinated birds survived infection with virulent viruses, and shed virus for the length of the experiment, 56 days.

A transmission experiment was done to determine if shed virus could infect other birds. The authors infected vaccinated or unvaccinated birds and asked if sentinel, unvaccinated chickens became infected. Unvaccinated birds died within 10 days after infection with virulent MDV, and did not transmit infection. In contrast, vaccinated birds survived at least 30 days, and co-housed sentinel animals became infected and died.

The experiments are well done and the conclusions are clear: more virulent Marek’s disease viruses replicate longer in vaccinated than unvaccinated chickens, and can be readily transmitted to other chickens. But these results do not prove that more virulent MDV arose because of permissive vaccines. Nor do the results prove in general that leaky vaccines lead to selection of more virulent viruses. The results simply show that a vaccine that does not prevent replication will allow transmission of virulent viruses.

To prove that vaccinated chickens can allow the selection of more virulent viruses, vaccinated chickens could be infected with an avirulent virus, and the shed virus collected and used to infect additional, vaccinated birds. This process could be repeated to determine if more virulent viruses arise. While the results of this gain-of-function experiment would be informative, they would be done in a controlled laboratory setting which would not duplicate all the selective forces present on a poultry farm.

The authors note that most human vaccines do prevent replication of infecting virus. They do not mention the one important exception: the Salk poliovirus vaccines. People who are immunized with the Salk vaccine can be infected with poliovirus, which will then replicate in the intestines, be shed in the feces, and transmitted to others. This behavior has been well documented in human populations, yet the virulence of poliovirus has not increased for the 60 years during which the Salk vaccine has been used.

I do not feel that these experimental results have general implications for the use of any animal vaccine. It is unfortunate that the work has been covered in many news sources with the incorrect implication that vaccines may be responsible for the emergence of more virulent viruses.